(1) Field of the Invention
The present invention relates to a constant-voltage circuit for a reference voltage.
(2) Description of the Related Art
In recent years, radio communications with higher transmission rates and higher speeds have been requested in mobile communication systems of, e.g., cellular phones. As digital modulation signals for achieving higher transmission rates and higher speeds, a digital modulation signal of high speed downlink packet access (HSDPA) system and a digital modulation signal of high speed uplink packet access (HSUPA) system have been proposed. In these digital modulation signals, the amplitude of the peak voltage tends to increase. Therefore, in a mobile communication system using these digital modulation signals, low distortion and high efficiency characteristics are necessary and thus a linear RF power amplifier is used as an amplifier for transmission. In such a linear RF power amplifier, an idle current has to be stabilized to keep linearity and the stabilization of an idle current requires a constant-voltage circuit. In recent years, constant-voltage circuits for stabilizing idle currents have been configured on the same integrated circuit (IC) as GaAs linear amplifiers.
FIG. 6 shows a constant-voltage circuit of the related art described in U.S. Patent Publication No. 2007/0159145. As shown in FIG. 6, the constant-voltage circuit of the related art includes a bipolar transistor Q1, a field-effect transistor Q2, two resistors R1 and R2, and a diode D1. A stable constant voltage Vreg is supplied from the source of the field-effect transistor Q2.
In the constant-voltage circuit of FIG. 6, the constant voltage Vreg is expressed by equation (1):
                              [                      Expression            ⁢                                                  ⁢            1                    ]                ⁢                                                                                                V          reg                =                                                            I                1                            ×                              R                2                                      +                          V              be                        +                          V              F                                =                                                                      R                  2                                                  R                  1                                            ×                              V                th                                      +                          V              be                        +                          V              F                                                          (        1        )            
where I1 is a current passing through the resistor R2, Vbe is the base to emitter voltage of the bipolar transistor Q1, VF is the leading edge voltage (forward voltage) of the diode D1, and Vth is the threshold voltage of the field-effect transistor Q2.
As has been discussed, in the constant-voltage circuit of the related art, the constant voltage Vreg is supplied from the source of the field-effect transistor Q2. In the constant-voltage circuit of the related art, however, the constant voltage Vreg considerably depends on the threshold voltage Vth of the field-effect transistor Q2 as is evident from equation (1). Typically, the threshold voltage Vth of the field-effect transistor is largely deviated during manufacture (manufacturing variations) and thus in the constant-voltage circuit of the related art, there are large manufacturing variations in the constant voltage Vreg due to deviations of the threshold voltage Vth of the field-effect transistor during manufacture.